CONSIDERED WITH REFERENCE TO HORTICULTURE. 



85 



other plants, may be made to grow on a base of two feet, and trained so as 

 to taper on the sides regularly to a top not thicker than an inch, at a height 

 of six or eight feet from the ground. Such hedges would fomi an elegant 

 and most effective shelter, provided they were at all times neatly kept. The 

 subject of shelter and exposure, however, in the open air is so well under- 

 stood, that any further observations seem unnecessary. 



266. The agitation of the air in plant structures has only lately been at- 

 tempted ; but, as a substitute for this, a partial renewal of the air, by opening 

 the sashes or ventilators of such structures, has long been in practice. This, 

 under many circumstances, particularly ui houses for tropical plants and for 

 forcing fruits, is very injurious to the plants, though it has been found impos- 

 sible to dispense with it to a certain extent. The injuries sustained by the 

 admission of the external air into a hothouse are greater or less according 

 to the difference of temperature, and, consequently, as we have seen (252), 

 of moisture. When the external air enters a hothouse in which the air is 

 at a high temperature, it rushes in with considerable velocity, driving out by 

 the pressure of the atmosphere the hot and vaporous au' by which the plants 

 are suiTounded, and becoming heated and charged with moisture, at the 

 expense of the earth in the pots and the foliage of the plants (270). 



267. The only complete remedy for this evil is to lieat the air before it is 

 admitted among the plants, by causing it to pass through a system of heated 

 tubes ; and to saturate the heated air, as it passes through those tubes, with 

 moisture, by placing among them a number of vessels of water. As this 

 mode, however, is somewhat difficult and expensive in the attainment, a 

 better practice is to put the air of the house in motion, admitting to it only 

 occasionally a small portion of the external air. This is done in a very 

 satisfactory manner by the mode of heating recently introduced by Mr. Penn. 

 By this mode the air is continually circulating from one side of the house to 

 the other, ascending on one side and 

 descending on the other, from back 

 to front ; one half of every revo- 

 lution being among the plants, and 

 the other half through a drain or 

 tunnel under the floor, the bottom 

 of which is covered with water, 

 which, by the heated air passing 

 over it, is kept at the same tem- 

 perature as that of the house. Fig. 

 2 is the section of a house heated 

 in Mr. Penn's manner, in which a Fig. 2. Section of a Hothouse heated hy not water 

 is the chamber contaming the heat- according to Mr. Penn's manner. 



ing pipes ; h a small opening in the front wall for occasionally admitting fresh 

 air ; c the drain from back to front, having the bottom covered with Avater, 

 through which drain the air passes, as shown by the direction of the 

 arrows in the figure. 



268. A sensible effect on the human feelings, produced by the atmosphere 

 of hothouses heated according to Mr. Penn's principle, is, that a high tem- 

 perature, say of 80° or 90°, can be breathed in as agreeably, and for as long a 

 period, as one of 60° or 70° not in motion. This result is partly attributed to 

 the motion given to the au' ; since, in the hottest days of summer, the heat 

 which would be oppressive in still air, is rendered not only bearable but 

 even agreeable, if the air is put in motion by a breeze. In like manner the 



